We proposed a novel temperature sensor based on the phenomena that the phase difference between principal polarization states in an optical retarder is a function of temperature. The polarization state of an optical beam is changed as it passes through the optical retarder, with the change dependent on the temperature. The temperature of the optical retarder is determined by comparison of the power difference between principal polarization states. A temperature sensor was successfully implemented using a polarization maintaining fiber of length 100 mm as the optical retarder. The change rate of phase difference on temperature was 0.236$^{circ}C$$_{-1}$ and the measurement error was $pm$0.038$^{circ}C$ over the temperature region of -2.6$^{circ}C$~3.4$^{circ}C$.